Be sure you're equipped with your running shield, and have alcohol in your inventory so you get drunk (Dwarven Ale is excellent, as one Ale only costs 100g). Please note that the benefits of buying alcohol to run to the forest outweighs the cost of the alcohol itself, and will actually benefit you.

In order to get to the forest in Fissure of Woe, you'll need to do a couple things. First off, when you first enter FoW, back up to the wall. This will (almost) ensure that you don't aggro the Shadow Army and get Rastigan the Eternal killed, thus kicking you back to your port of entry. Now, you'll need to run to the right, staying along the back wall. You will more than likely aggro the shadow army closest to you on the right - make sure to cast Drunken Master. This will help you get past them and if you keep going along the wall, you will just barely miss aggro'ing another group of 3 from the shadow army. After this you'll need to leave the wall and run to the left, behind the 3 shadows and towards what looks like an open spot. Be careful as many patrols are close by, and it is advised not to aggro them.

After you have run a little ways, you will be on a plateau in between 2 valleys. Stay on the plateau all the way until it ends. Near the end, a Shadow Beast will appear. Continue running straight until you hit a large cliff wall. Stop here (as long as no monsters are following you) and heal. After you get here, turn right and run along the wall until it bears off to the left. There is a little straight here that you will want to take. You will most likely aggro another group here, but Drunken Master should be able to outrun them. Run along the wall again until it juts out to the right. Now comes the tricky part. The 3 shadows up ahead aren't as big a deal as the skeletons at the bottom of the hill to your right. If these two groups are far apart, you may be able to run through the shadows and by keeping left, reach a very tiny ledge that will allow you to rest before taking on the skeletons. If not, you will need to try and aggro the skeletons into the shadows, as they will fight each other, letting you get away. To do this, run towards the skeletons at normal speed. Shoot one of them with an arrow (using your longbow), in order to aggro them, then draw them up the hill, running with your run skill activated. This is crucial because if you stay within range of the spell caster in their group, you could be hit with Deep Freeze, and not being able to move in this part of the run equals a long walk off a short pier. Once they start fighting or you have run around the skeletons, stay left, hugging the wall at the bottom of the hill. Here you can rest again, letting everything recharge and waiting for a patrol of skeletons to separate from the larger group ahead of you. You can either try and stay on the wall, sneaking past the larger group; or you can wait for the patrol to leave, and run around the skeletons. It really depends on how they spawn. Either way, once you are past that group, you are effectively in the forest.